A MEMS (Micro Electro Mechanical Systems) device formed by a movable electrode and fixed electrode has features of low loss, high insulation performance, and linearity. Hence, a MEMS device such as a capacitor, a switch, or an acceleration sensor is attracting a great deal of attention as a key device for a next-generation portable cellular phone.
A feature of a MEMS device lies in that it has a mechanically movable portion (MEMS element), unlike a normal semiconductor device. Therefore, a MEMS device cannot be directly packaged on a circuit substrate, and the packaging operation requires forming a protective cover having a cavity. Especially, in terms of the cost and size, it is very effective to form a protective cover (thin-film dome) using the same process as that of manufacturing a MEMS device.
There is provided a method of manufacturing a MEMS element having a movable portion, and a thin-film dome structure which stores the MEMS element, that includes the step of forming and patterning a first sacrificial layer, the step of forming a MEMS element on the first sacrificial layer, the step of forming a second sacrificial layer on the first sacrificial layer and MEMS element, and patterning it, the step of forming a first layer (for example, an SiOX film) on the second sacrificial layer, the step of forming through holes in the first layer, the step of removing the first sacrificial layer and second sacrificial layer via the through holes, the step of forming a second layer which closes the through holes on the first layer, and the step of forming a third layer (for example, an SiN film) having a moisture-proof property on the second layer. The first, second, and third layers form a thin-film dome having a cavity.
In forming this thin-film dome structure, when the thin-film dome deforms due to expansion of a cavity portion, a crack is generated in a stress concentration portion due to the deformation. This may degrade the sealing performance of the dome interior, thus degrading the reliability of the device.